2.5. Defining Colorspaces in V4L2

In V4L2 colorspaces are defined by four values. The first is the colorspace identifier (enum v4l2_colorspace) which defines the chromaticities, the default transfer function, the default Y’CbCr encoding and the default quantization method. The second is the transfer function identifier (enum v4l2_xfer_func) to specify non-standard transfer functions. The third is the Y’CbCr encoding identifier (enum v4l2_ycbcr_encoding) to specify non-standard Y’CbCr encodings and the fourth is the quantization identifier (enum v4l2_quantization) to specify non-standard quantization methods. Most of the time only the colorspace field of struct v4l2_pix_format or struct v4l2_pix_format_mplane needs to be filled in.

On HSV formats the Hue is defined as the angle on the cylindrical color representation. Usually this angle is measured in degrees, i.e. 0-360. When we map this angle value into 8 bits, there are two basic ways to do it: Divide the angular value by 2 (0-179), or use the whole range, 0-255, dividing the angular value by 1.41. The enum v4l2_hsv_encoding specifies which encoding is used.

Note

The default R’G’B’ quantization is full range for all colorspaces except for BT.2020 which uses limited range R’G’B’ quantization.

v4l2_colorspace

V4L2 Colorspaces

Identifier	Details
V4L2_COLORSPACE_DEFAULT	The default colorspace. This can be used by applications to let the driver fill in the colorspace.
V4L2_COLORSPACE_SMPTE170M	See Colorspace SMPTE 170M (V4L2_COLORSPACE_SMPTE170M).
V4L2_COLORSPACE_REC709	See Colorspace Rec. 709 (V4L2_COLORSPACE_REC709).
V4L2_COLORSPACE_SRGB	See Colorspace sRGB (V4L2_COLORSPACE_SRGB).
V4L2_COLORSPACE_ADOBERGB	See Colorspace Adobe RGB (V4L2_COLORSPACE_ADOBERGB).
V4L2_COLORSPACE_BT2020	See Colorspace BT.2020 (V4L2_COLORSPACE_BT2020).
V4L2_COLORSPACE_DCI_P3	See Colorspace DCI-P3 (V4L2_COLORSPACE_DCI_P3).
V4L2_COLORSPACE_SMPTE240M	See Colorspace SMPTE 240M (V4L2_COLORSPACE_SMPTE240M).
V4L2_COLORSPACE_470_SYSTEM_M	See Colorspace NTSC 1953 (V4L2_COLORSPACE_470_SYSTEM_M).
V4L2_COLORSPACE_470_SYSTEM_BG	See Colorspace EBU Tech. 3213 (V4L2_COLORSPACE_470_SYSTEM_BG).
V4L2_COLORSPACE_JPEG	See Colorspace JPEG (V4L2_COLORSPACE_JPEG).
V4L2_COLORSPACE_RAW	The raw colorspace. This is used for raw image capture where the image is minimally processed and is using the internal colorspace of the device. The software that processes an image using this ‘colorspace’ will have to know the internals of the capture device.

v4l2_xfer_func

V4L2 Transfer Function

Identifier	Details
V4L2_XFER_FUNC_DEFAULT	Use the default transfer function as defined by the colorspace.
V4L2_XFER_FUNC_709	Use the Rec. 709 transfer function.
V4L2_XFER_FUNC_SRGB	Use the sRGB transfer function.
V4L2_XFER_FUNC_ADOBERGB	Use the AdobeRGB transfer function.
V4L2_XFER_FUNC_SMPTE240M	Use the SMPTE 240M transfer function.
V4L2_XFER_FUNC_NONE	Do not use a transfer function (i.e. use linear RGB values).
V4L2_XFER_FUNC_DCI_P3	Use the DCI-P3 transfer function.
V4L2_XFER_FUNC_SMPTE2084	Use the SMPTE 2084 transfer function.

v4l2_ycbcr_encoding

V4L2 Y’CbCr Encodings

Identifier	Details
V4L2_YCBCR_ENC_DEFAULT	Use the default Y’CbCr encoding as defined by the colorspace.
V4L2_YCBCR_ENC_601	Use the BT.601 Y’CbCr encoding.
V4L2_YCBCR_ENC_709	Use the Rec. 709 Y’CbCr encoding.
V4L2_YCBCR_ENC_XV601	Use the extended gamut xvYCC BT.601 encoding.
V4L2_YCBCR_ENC_XV709	Use the extended gamut xvYCC Rec. 709 encoding.
V4L2_YCBCR_ENC_BT2020	Use the default non-constant luminance BT.2020 Y’CbCr encoding.
V4L2_YCBCR_ENC_BT2020_CONST_LUM	Use the constant luminance BT.2020 Yc’CbcCrc encoding.
V4L2_YCBCR_ENC_SMPTE_240M	Use the SMPTE 240M Y’CbCr encoding.

v4l2_hsv_encoding

V4L2 HSV Encodings

Identifier	Details
V4L2_HSV_ENC_180	For the Hue, each LSB is two degrees.
V4L2_HSV_ENC_256	For the Hue, the 360 degrees are mapped into 8 bits, i.e. each LSB is roughly 1.41 degrees.

v4l2_quantization

V4L2 Quantization Methods

Identifier	Details
V4L2_QUANTIZATION_DEFAULT	Use the default quantization encoding as defined by the colorspace. This is always full range for R’G’B’ (except for the BT.2020 colorspace) and HSV. It is usually limited range for Y’CbCr.
V4L2_QUANTIZATION_FULL_RANGE	Use the full range quantization encoding. I.e. the range [0…1] is mapped to [0…255] (with possible clipping to [1…254] to avoid the 0x00 and 0xff values). Cb and Cr are mapped from [-0.5…0.5] to [0…255] (with possible clipping to [1…254] to avoid the 0x00 and 0xff values).
V4L2_QUANTIZATION_LIM_RANGE	Use the limited range quantization encoding. I.e. the range [0…1] is mapped to [16…235]. Cb and Cr are mapped from [-0.5…0.5] to [16…240].